I looked up Eaton the other day - and they sure are proud of their springs - whew! The material they claim to use is pretty much universal - so rocket science isn't what they are selling. One of the local HAMB guys has suggested Arizona Spring as a place to "shop" here in Tucson, so I may go over there next week. Using a local source could be the best idea anyway.
Many of the web sources I've found all use the same part numbering scheme for the bird springs - so I'm thinking there is a "root source" vendor somewhere. Might take a while to find them....but it could net some better pricing (and more specific information). No one seems to know what the load ratings of the various varieties are.
I did grovel under the car yesterday, long enough to find a "fifth" leaf on my outfit - but the tiny fifth leaf doesn't look to be a major load carrying or stiffening member.
Steve Metzger Tucson, Arizona